Counting tablet PCs as personal computers, Apple is about to overtake Hewlett-Packard and become the world’s top personal computer vendor. All should be known soon when Apple reveals holiday quarter earnings in a conference call with analysts scheduled for Tuesday, Jan. 24.

HP CEO Meg Whitman was first to admit that such a turn of event neither would nor be entirely unsurprising given Apple’s lead in the post-PC world. Whitman said back in November, “It’s possible if you integrate tablets.”Fortune’s Philip Elmer-DeWitt wrote that, based on a poll of 42 analysts, iPad sales could hit the 14 million mark, a notable increase over the 11.12 million iPads sold during the September quarter.

Fortune’s estimates range from 11.7 million (Hendi Susanto of Gabelli & Co) to 19.47 million iPads (Alexis Cabot of the Apple Finance Board). According to research firm Gartner, HP shipped 14.7 million PCs in the last quarter, down 16 percent from a year ago.

Now, Apple in this last quarter cleared 4.89 million Macs, and its holiday sales are estimated to exceed 5 million units. Combined, iPad and Mac will have sold over 20 million units during the holiday quarter, enough to give the Cupertino, Calif.-based technology powerhouse a few million units lead over HP, the world’s leading computer vendor.

Estimated unit sales translate into a 17.6-percent market share for Apple versus 13 percent for HP, representing a landmark achievement by any measure. Again, that’s assuming tablets are counted as personal computers. As noted by Asymco’s Horace Dediu, Apple has never held the top spot. Its Apple II system peaked at 15.8 -percent share in 1984 and the Mac peaked at 12 percent in 1992. Interesting that Lenovo CEO praised Apple last week, saying about Android on tablets that “We still need to learn something, we still need to improve something”.